The invention generally relates to electronic circuits, and more particularly to voltage controlled oscillators (VCOs).
VCOs are well known devices and have been used in many applications, e.g., in generating frequency signals for networking and communication systems. Most VCOs are single ended or differential CMOS, e.g, single ended CMOS VCOs, differential CMOS VCOs and bipolar VCOs. As networking and communication systems are playing increasingly more important roles in the present information age, demands for high speed VCOs are significantly increased to keep up with the speed of these systems. High speed VCOs typically are implemented exclusively with bipolar transistors. These VCOs, however, suffer from serious drawbacks. In particular, the outputs of these VCOs do not provide consistent performance. These VCOs change their output frequency swings depending on the speed of the input signals. Such variations at the output frequency swings seriously affect the overall performance of circuits in which these VCOs are used.
Therefore, there is a need for an improved high speed VCO that delivers consistent output performance.
The invention provides an improved high speed VCO. According to one embodiment of the invention, a VCO buffer cell is provided. The cell comprises a differential pair of transistors having input terminals for receiving input signals and output terminals for providing differential voltage swing in response to the input signals; and a current mirror circuit, operably coupled to the pair of transistors, that is configured to receive a first external reference current and provide a mirrored current to an active one of the transistors. The differential voltage swing has a frequency which is determined based on the reference current.
According to one aspect of a specific embodiment of the invention, the current mirror circuit of the cell includes a first current mirror that is configured to receive the first external reference current and provide the mirrored current; and a second current mirror, operably coupled to the first current mirror, that is configured to receive the mirrored current, provide it to the active one of the transistors and provide a reference voltage such that the output of the active transistor is equal to a mirrored value of the reference voltage. The current mirror circuit allows self-regulation of the output frequency swings of the VCO buffer cell, without regard to the changes in the speed of the input signals received by the cell. This ensures consistent performance at the cell output.
According to another aspect of the specific embodiment of the invention, the pair of transistors of the cell is bipolar transistors, and the current mirror circuit is composed of MOS transistors.
According to a further aspect of the specific embodiment of the invention, the cell further comprises two emitter followers each operably coupled to the output of a respective one of the transistors for providing voltage level shifting to the differential voltage swing.
According to a still further aspect of the specific embodiment of the invention, the cell further comprises two capacitive circuits each coupled between the output of a respective one of the transistors and a predetermined voltage level for allowing adjustment of the frequency of the differential voltage swing and for providing voltage clamping to the outputs of the transistors.
The invention also provides a multi-stage ring oscillator having a plurality of the VCO buffer cells.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.